IRS / Wilwood questions

[FDLWproject]

Hi everyone -

I've got a few questions as I wrap up the IRS on my roadster. I've got the 2015 mustang IRS, and Wilwood 12.88 brake kit with separate brake and parking brake calipers.

Question 1 - Yes I'm prepared for you all to laugh. I've seen a number of comments about checking the rear wheels/hubs for spin after getting everything aligned and brakes attached. I've gotten everything assembled including the brake calipers and rotor alignment is great. No rubbing etc. and rotors are temporarily attached with lug nuts to ensure rotor is vertically aligned. However, I am having trouble getting the rotor to spin without putting a little muscle into it. I can get it to move, and I see the pinion flange spinning on the differential, but it just seems like there is a lot of resistance. Checked with some of my smarter engineer family and they don't seem concerned. It is likely driven by the different in reverse force being applied at the wheel instead of the driveshaft side, and it having to work against the normal gear step-down. Any thoughts on this or am I over-thinking it?

Question 2 - The Wilwood parking brake assembly has been a bit tricky. I can't seem to get the piston retracted enough to slide it over the rotor for assembly. I did find a thread below and used the same approach to fill it with fluid / remove air. It does retract the piston, but it doesn't seem to be as "flush" with the caliper as it needs to be. Any suggestions on other adjustments to get it to retract more? Did I put to much fluid in it or do I need to adjust the nut/threaded rod end?

#9 post here - https://thefactoryfiveforum.com/showthread.php?43769-Wilwood-Park-Brake-Issue

Thanks!

[Windsor]

Regarding (1)...

Sounds like a limited-slip differential. How easy is it to spin the pinion flange by hand?

If you have an open differential, spin one side and the other side spins opposite rather freely. The 1:1 spider gears and side gears are much easier to move than trying to make the 3.55:1 (give or take) pinion gear turn.

With a limited-slip, that's no longer an option, you have to convince the pinion gear to turn. (some exceptions apply, e.g. Torsen)

Give the pinion flange a spin, see how easy it feels.

[FDLWproject]

Thanks Rob this is helpful. I tried from the pinion flange and it’s a bit easier. Still some resistance though.

[edwardb]

1. All 2015+ Mustang diffs are limited slip. Some the standard Ford Trac-Lock (clutch disks) and some are gear type (Torsen). If yours is the Trac-Lock type (most common) it's completely normal that it's hard to turn from one wheel through the diff to the other side and the driveshaft. Also normal when turning one wheel the other will turn in the opposite direction. Sounds like yours is fine. Those Super 8.8 IRS diffs are pretty bullet proof.

2. I've installed the separate e-brake Wilwood setup several times. I've not had any trouble with them fitting over the rotors. The thread you linked to is a little confusing to be honest. Not all the comments are about this type. Among other things, some talk about brake fluid. The separate e-brake assembly is 100% mechanical. I'm personally not aware of any adjustments on it. Doesn't mean there aren't any. Just haven't had to do anything with them. I'd call Wilwood and see what they suggest. Make sure they understand exactly what you have. Should look like one of these: https://www.wilwood.com/PDF/Flyers/fl305.pdf.

[AA-ron]

I just finished my build this summer and have the same IRS set-up. I had the same exact experience you are describing with the rear rotors being very difficult to turn. I assumed it was the brakes dragging a bit. In any case, after the first drive everything loosened up and now they spin relatively freely. I just turned 2k on it, and all is great.

[edwardb]

What I described about trying to turn the rear wheels will be somewhat stiff with or without rear brakes installed. Disk brakes by design drag slightly even when not applied. Try spinning the front tires where the only drag will be the brakes and you should hear the pads lightly contacting the rotors. While the fronts will typically turn several revolutions if given a good push, they will never be as free as they might be without the brakes installed. The rears, with or without brakes, will never spin freely like that. You have to grab the tire and position it where you want. All normal.

[CraigS]

I agree w/ Edwardb. Remember that in addition to your thought on the diff gears (rotating a wheel you are trying to turn the driveshaft about 3 times the speed you turn the wheel) w/ the lim slip when your turn one wheel, you are also turning the other wheel, the diff, the driveshaft, and some of the gears in the trans. Also the ring gear is sitting in maybe 2 inches of thick gear oil so that is like stirring molasses w/ a spoon.

[FDLWproject]

Thank you everyone for the detailed responses. This is really helpful. Sounds like I’m not having any issues with the wheel rotation or IRS, but I’m going to call the Wilwood support team on the parking calipers. I still can’t seem to get the piston retracted enough to get over the caliper without it being completely clamped onto the rotor.

Side note - I’ve seen a lot of complaints about the strength of the parking brake hold from Wilwood. One of the suggestions was to replace the compression spring with a stronger one, so I figure I’ll go ahead and do that before getting everything installed. NAPA sells a 40# compression spring for cheap.